The trial of external quality assessment for inborn error of metabolism was performed in 2006. Total 12 specimens for the conventional newborn screening tests were distributed to 17 laboratories. The response rates were 100%(17/17). All the control materials were sent as filter paper forms. Each laboratory replied the test results as the screening items they were doing as a routine test at the reception of the specimen among PKU screening, TSH, T4 (total/free), galactosemia screen, maple syrup urine disease screen and homocytinuria screen. The mean, SD, CV, median and range were analyzed. From 2004 trial, pilot proficiency test for newborn screening using tandem mass spectrometry (MS/MS) has been also performed. In this trial, total 8 blood spots were distributed to 9 laboratories performing newborn screening using MS/MS. Participants results for amino acids & acylcarnitine and their cutoff values were analyzed.